Foundry core fabrication process



g- 6, 1966 E. B. DUNNING ETAL 3,266,108

FOUNDRY CORE FABRICATION PROCESS Fixed Jan. 8, 1964 SAND J RESINSOLUTION MIX OFF I RESIN SOLVENT 4 DRY FREE-FLOWING RESIN-COATED SANDMOLD

VACUUM 4 RES'N SOLVENT SO LVATE OFF RESIN SOLVENT 4 VACUUM RESIDUALFLUSH SOLVENT OPEN MOLD INVENTORS CORE EDWARD B. DUNNING ROGER H. K TKEATTORNEY United States Patent 3,266 108 FOUNDRY CORE FAliRICATIONPROCESS Edward B. Dunning, Minneapolis, Minn., and Roger H.

Kottke, Hatboro, Pa., assignors to Archer-Daniels-Midland Company,Minneapolis, Minn., a corporation of Delaware Filed Jan. 8, 1964, Ser.No. 336,376 12 Claims. (Cl. 22-493) This invention relates to theProduction of foundry cores and molds.

Many types of binders have been proposed for foundry cores and molds.The use of numerous organic binders is well known. For many years,linseed oil and its derivatives were the most popular foundry corebinders. More recently, synthetic binders have been developed which haveadvantages in speed of cure and greater tensile strength. Normally, theorganic resins used for binding foundry cores and molds arethermosetting resins. Thermoplastic resins have been suggested but arenormally not used because of the difliculty in obtaining reproducibledimensional accuracy in the core. A thermoplastic binder allows the coreto sag during curing or in removing a hot core from its mold.

The foundry trade is constantly striving to improve the quality anddimensional accuracy of their castings. The accuracy and precision withwhich a casting is made is dependent on the accuracy obtained in thecore. The best accuracy is obtained by curing or hardening the corewhile it is still in the mold. Several methods have been suggested forhardening the sand while still in the mold. One method is to gas analkali silicate with carbon dioxide, another method is to cure in aheated mold, and a further method is to cure the resin catalyticallywhile in the mold.

Each of these methods have their advantages and disadvantages. Thealkali silicate binders produce good dimensional accuracy; but becausethey are inorganic, cores formed using these binders do not have thedesired breakdown characteristics that are necessary to promote aneflicient foundry operation. The curing of cores in a heated mold is arapid operation but requires somewhat of a sacrifice in dimensionalaccuracy. Since the mold is normally heated to about 450 F., the sand isin an expanded state while in the pattern; and on cooling, because ofcontraction, the core is not an exact replica of the mold. Dimensionalaccuracy of the catalytically cured cores is good and the breakdown isalso good since the binder is normally an organic material. Suchbinders, however, have disadvantages in that the prepared sandcomposition must be used immediately. This means that sand mixes must beprepared individually for each molding operation.

It is therefore an object of this invention to provide a novel method ofimproving the dimensional accuracy of foundry cores and molds by a novelin situ hardening process. Another object of this invention is toprovide a method of forming hard foundry cores and molds from sandprecoated with a thermosetting resin without thermosetting the resin. Afurther object of this invention is to provide a means for formingfoundry cores and molds from free-flowing synthetic resin-coated sandwithout the use of heat.

The objects of this invention are accomplished by a process whichcomprises precoating sand with a synthetic organic resin, drying theprecoated sand to a free-flowing state, forming the prepared sand in amold, solvating the resin with a solvent, and thereafter removing thesolvent.

This process overcomes many of the disadvantages of the prior art.Dimensional accuracy is of the highest order since the core is cured atroom temperatures while in the mold. The binder is organic and thereforepro- "ice vides rapid breakdown at casting temperatures. In addition,the prepared sand is in a dry free-flowing state and can be stored forindefinite periods. Thus, the more desirable characteristics of foundrybinders and molding operations are incorporated into a single systemwhich greatly facilitates the production of metal castings of highdimensional accuracy.

The process of this invention provides two methods of accomplishing thebonding of the core sand. One method is to shape the core by placing adry synthetic resincoated sand into a cavity, pressurizing with agaseous sol- .vent and establishing liquid-vapor equilibrium topartially dissolve the resin coating allowing it to unite at the pointsof contact, releasing the pressure to flash off the solvent, andsubsequently removing the bonded core shape from the cavity. The secondmethod is to shape the core by placing a dry synthetic resin-coatedsand, into a cavity, solvating the resin coating with a volatile liquidsolvent, flashing off the solvent under reduced pressure, andsubsequently removing the bonded core shape from the cavity.

The accompanying single figure of the drawing illustrates thisinvention.

The term foundry core when used means both cores and molds, thedifferentiation being that cores are normally internal structures andthe molds are external structures. The term solvent means both normallyliquid and normally gaseous solvents which are capable of plasticizing aresin. The term solvating means the process of dissolving, plasticizing,or softening a resin with a solvent. Solvating to a liquid state meansplasticizing sufficiently to cause flow.

The synthetic resins useful in the process of this invention are thoseresins which are capable of curing to a rigid state. In addition, theuseful resins are those resins which are normally solids when notplasticized and have softening points above F.

The term synthetic resin includes both thermosetting and thermoplasticresins. The preferred resins, however, are thermosetting resins. Resinsused in the process of this invention are plasticized with sufiicientvolatile solvent to obtain a free-flowing liquid having a viscosity ofless than about 30 stokes. This is accomplished by adding variousamounts of solvent, depending on the polymerized state of the resin.

The term thermosetting resin is used herein to distinguish thecharacteristics of these resins from those of the thermoplastic resins.The distinguishing characteristic of thermosetting resin is not thecuring with heat but the curing to an infusible state by thecrosslinking of a polymer. After a polymer is crosslinked, it is veryresistant to solvent and heat. Thermosetting resins are often cured atroom temperatures by the addition of a catalyst.

In carrying out the processes of this invention the aggregate materialis first coated with a synthetic resin having a softening point above100 F. The aggregate material is normally a lake sand or a silica sandbut may also be 1any other inexpensive heat-resistant aggregate matenaThe preferred resins are thermosetting resins such as hydrocarbonresins, alkyd resins, rosin ester resins, rosin ester modified resins,novolak resins, and acrylic resins. In addition to the thermosettingresins, thermoplastic resins having softening points above 100 F. can beutilized because the molding process is conducted without the use ofheat which would normally cause distortion in removing from a heatedpattern or an oven. During the casting operation, sagging is eliminatedbecause of the high metal temperature which carbonizes the resin binderto a rigid state faster than the sand can flow.

The hydrocarbon resins useful with this invention are the polymen'zableresins obtained from petroleum fractions. These resins are bothaliphatic and aromatic in structure and are distinguished by theircapability of polymerizing under heat, pressure, or by catalyticreagents such as Friedel-Crafts catalysts or strong acids. The usefulhydrocarbon resins are polymerized to a normally solid state having asoftening point above 100 F. By further heating, these resins willpolymerize to higher polymers.

The alkyd resins useful with this invention are of the medium-oil andshort oil alkyd type. Such resins are the reaction product of a fattyester and a dibasic acid such as maleic and phthalic. These resins mayalso be modified with hydrocarbon resins by blending with polymerizedhydrocarbon resins or by copolymerizing with unsaturated hydrocarbonmonomers such as cyclopentadie ne and dicyclopentadiene.

The fatty esters used in alkyd resins are normally derived from naturaloils. Synthetic fatty esters are also useful. They are reaction productsof polyhydroxyl alcohols such as pentaerythritol glycerine and sorbitoland carboxylic acids of 8 to 26 carbon atoms. The desirable fattyesters, also known as fatty oils, are characterized by ethylenicunsaturation in a hydrocarbon chain of 8 to 26 carbon atoms. Such oilsare derived from animal, vegetable, and marine sources.

The rosin ester resins which are useful with this invention are estersobtained by esterifying tall oil rosin, gum rosin, or wood rosin with apolyhydroxy alcohol to form a resin which has a softening point above100 F. Polyhydroxyl alcohols used include pentaerthyritol, glycerine,and sorbitol and the like. Included with these rosin esters are themodified rosin resins such as those modified with 1% to about 40%phenol, 1% to 70% fatty ester, or various dibasic acids such as maleicand phthalic.

The phenolic resins useful in this invention are of the novolak type.Novolak resins are the condensation product of phenol and formaldehydeunder acidic conditions. Such resins are phenol-rich in that a furtheraddition of formaldehyde causes cross-linking. The novolak type resinsuseful are those which have been polymerized to a softening point above100 F. In the operation of this invention, using a novolak resin, anadditional amount of formaldehyde is added to the sand-resin mix duringthe coating of the sand. The formaldehyde is normally added in the formof hexamethylenetetramine which on heating provides free formaldehydefor further polymerization and crosslinking of the phenolic resin.

Acrylic resins are also useful in this invention. The resins useful arepreferably thermosetting compositions. Such compositions are composed oflinear polymers and crosslinking monomers which have functionality in anamino group, an amido group, a hydroxyl group, a carboxyl group or aglycidyl group. Typical of this type of polymer is a linearthermoplastic copolymer which is crosslinked by an epoxy oil to athermosetting composition. A typical linear polymer is a copolymer of anaromatic vinyl monomer, a lower alkyl ester of acrylic acid, and acrylicacid. The copolymer is diluted with an inert solvent in admixture withan epoxy oil. Curing is initiated by a free radical catalyst or heat.Numerous other systems are well known deriving their functionalitythrough the functional groups named.

The resins described are soluble in aromatic and aliphatic solvents.Depending on the structure of the resin, volatile solvents such asmethanol, ethanol, isopropanol, diethyl ether, petroleum naphtha,petroleum ether, benzene, xylene, toluene, acetone, chloroform,methylene chloride, chlorinated hydrocarbon solvents, ketones, esters,ethers, and the like and combinations thereof are used to reduce theviscosity of the resin prior to the coating operation. Thesolvent isremoved during the coating operation to yield a dried but uncured resin.

As has been stated, the thermosetting resins are the preferred polymers.During the process of coating the foundry sand, the coating resin is notcured to an infusible state. The coating is merely dried on the sandgranular by evaporation of a volatile solvent. This procedure allowssubsequent solubilizing and plasticizing of the coating.

The amount of resin used for coating the sand is from 1% to about 8% byweight based on the weight of sand. The preferred range is about 1% toabout 4% resin. With thermosetting resins, a polymerizing agent ispreferably added during the resin coating operation. Subsequent heatingeither prior to casting or curing during the metal pouring operationwill cure the thermosetting resin to an infusible state.

Both liquid and gaseous solvents are utilized to resolvate theresin-coated sand. Solvents which are normally liquid are preferred. Theliquid solvents preferred are solvents capable of solubilizing the resincoating and which readily volatilize in vacuums of 20 to 30 inches ofmercury at temperatures of about 20 C. to 50 C. Preferably solvents suchas chloroform, petroleum ether, petroleum naphtha, acetone methylenechloride, diethyl ether, ketones, esters, and chlorinated hydrocarbonsare used to solvate the resin coating. Solvents which are normally gasesmay also be used but are not preferred.

In forming cores using normally gaseous solvents, the operation iscarried under sufficient pressure to produce a liquid-vapor equilibriumwithin the core shape. Absolute pressures of about 10 to about 5,000pounds per square inch are used. The solvent is then released underatmospheric pressure. The preferred method is to solubilize the resinwith a normally liquid solvent and subsequently remove the solvent under20 to 30 inches of mercury vacuum.

The amount of solvent required for the solvation procedure of thisinvention is dependent on the type of resin, the amount of resin coatedon the sand and the solvating power of the solvent. The amount ofsolvent used is of little economical concern since more than 98% to 99%of the solvent can be recovered and reused. The solvent required forsolvation is normally between a weight ratio of 1:200 to 1:10 solvent toresin-coated sand. The requirement is that sufiicient solvent be used tosolvate the resin coating sufiiciently to cause resin flow at the pointsof sand contact but not enough solvent to cause removal of the resin isused. Extraction of the resin has not caused difficulties.

During the operations of this process the evaporation or volatilizationof the solvent causes a temperature change which in some situationsslows down the processing time. Low temperatures inhibit resin flow andrequire larger quantities of solvent to accomplish the solvation.Therefore, it is desirable to maintain a relatively constant temperatureof about 20 C. to about 50 C. in the pattern during the moldingoperation. This is readily accomplished by means of a water jacket orother heat stabilizing means.

The invention will be better understood with reference to the followingexamples which are illustrations of certain preferred embodiments of thepresent invention. Unless otherwise indicated, all parts and percentagesused herein are by weight.

Example I A resin-coated sand was prepared by coating American FoundrySociety (APS) Standard Sand with a phenol formaldehyde resin of thenovolak type having a melting point of 107 C. This resin was diluted to50% solids with ethanol. The sand was coated by mixing parts of resinsolution and 9 parts of hexarnethylenetetramine with 2000 parts of sand.The solvent was evaporated during the mixing resulting in a free-flowingsand coated with 3% resin solids.

The prepared sand was placed in a mold which was permeable to gases andliquids. The mold was filled with the resin-coated sand and placed in achamber Which was evacuated to 29 inches of mercury vacuum. Methylenechloride was added to the evacuated chamber. This addition increased thepressure to atmosphericpressure. The chamber temperature was maintainedat 30 C. during the solvation procedure. The chamber was thenre-evacuated to 29 inches of mercury vacuum and flushed with air back toatmospheric pressure. For a core weighing 190 grams, 40 milliliters ofsolvent was used. More than 98% of the solvent was recovered andrecycled in subsequent solvating procedures.

Cores formed in this manner were ready for immediate use in the castingoperation. The high temperatures of molten metal instantaneouslythermosets or carbonizes the resin binder eliminating sand movement .orflow. Castings formed using cores made by the process of this inventionhad high dimensional accuracy.

Example II Resin-coated sand prepared as in Example I was blown into amold which was maintained under reduced pressure of about 27 inches ofmercury vacuum. The sand contained in the pattern was then gased withmethylene chloride by injecting 'liquid methylene chloride into the moldchamber. The liquid methylene chloride vaporized under the reducedpressure causing the pressure to increase from about 27 inches ofmercury vacuum to atmospheric pressure. Reduced pressure was againapplied drawing the vaporized methylene chloride through the sand-filledcavity. The methylene chloride in a liquidvapor'equilibrium, solvatedthe resin which coated the sand sufficiently to allow the resin to flowand bond at the points of contact. The solvent was drawn off undervacuum and recovered for subsequent solvations. The mold chamber wasthen flushed with air by increasing the pressure to atmospheric.

Cores formed in this manner were ready for immediate use in castingoperations.

Example 111 Resin-coated sand prepared as in Example I was rammed into amold permeable to gases and liquids. The pattern was constructed so thata vacuum could be drawn on the contents of the mold. The resin-coatedsand in the mold was hardened by drawing a vacuum of about 29'inches ofmercury and injecting sufiicient chloroform into the pattern so that thevacuum was reduced from 29 inches of mercury to about atmosphericpressure. The temperature of the mold was controlled by a water jacketat 30 C. to 35 C. so that the volatilizing chloroform would not reducethe temperature and thus retard vo'latilization and solvation. Thevacuum within the mold was again increased to about 29 inches of mercuryto remove the chloroform. The chloroform removed was condensed andretained for recycling.

With the vacuum again at about 29 inches of mercury, chloroform wasagain introduced allowing the pressure to return to atmospheric. Thechloroform was again removed from the mold by increasing the vacuum toabout 29 inches of mercury. The mold was then flushed with air allowingit to return to atmospheric pressure. The resin coated sand within themold was found to have been reduced to a hardened state. Cores formed inthis manner could be used immediately without further curing or in thealternative could be subjected to a heat cure to increase the hardness.

Example IV The process of Example HI was again repeated using diethylether as the solvent. Cores formed in this manner were suitable forimmediate use in casting operations.

Example V An acrylic resin was made by charging 65 parts xylene and 16parts butyl alcohol to a flask equipped with an agitator, thermometer,reflux condenser and a metering tank. The metering tank contained 50parts vinyl toluene, 10.2 parts methylmethacrylate, 6 parts Z-ethylhexylacrylate, 12.9 parts of rnethacrylic acid and 2.4 parts cumenehydroperoxide. The xylene and butanol were heated to 240 F. The monomermixture in the metering tank was then added slowly over a 3-hour periodwhile the reaction mixture was maintained at 240 F. The reaction mixturewas kept at about 240 F. for 3 hours after completion of the monomeraddition. At this time the percent nonvolatile of the mixture wasdetermined to be 50%, indicating the copolymerization reaction wascomplete. To form the thermosetting resin, 15 parts epoxidized soybeanoil having an oxirane value of 6.3% was added to parts of the copolymer.

A resin-coated sand was prepared by coating Nugent Lake sand with theprepared thermosetting acrylic resin. The acrylic resin had a solidcontent of about 57.5% which was reduced by an addition of xylene to 50%solids. The sand was coated by mixing 4 parts of resin solution with 100parts of sand. The solvent was evaporated during the mixing resulting ina free-flowing sand coated with 2% resin solids which was dried unto thesand granules. v

The prepared sand was placed in a mold which was permeable to gases andliquids and in which a reduced pressure could be applied. The mold wasevacuated to about 29 inches of mercury vacuum and methylene chloridewas added to the evacuated mold. The temperature of the mold wasregulated with a water jacket vso as to maintain a temperature between30 and 40 C.

The evaporation of the methylene chloride increased the pressure withinthe mold to atmospheric pressure. The mold was then re-evacuated toabout 29 inches of mercury. The methylene chloride was condensed andretained for subsequent solvation. The evacuated chamber was thenflushed with air to return to atmospheric pressure.

The cores formed in this manner were ready for immediate use in thecasting operation. Since the resin had not been polymerized to aninfusible state, a subsequent baking period could be used if desired toincrease the strength of the core. It, however, has been found that thisis not necessary since the high temperatures of molten metalinstantaneously carbonizes the binder to higher strengths.

Example VI Utilizing the resin-coated sand and procedure of Example V,foundry cores were prepared using diethyl ether as the solvent. Coresproduced in this manner were ready for immediate use in castingoperations.

Example VII Using the resin-coated sand and procedure of Example V,cores were again made utilizing chloroform as the solvent. Coresprepared in this manner could also be used immediately without furthertreatment in casting operations.

The procedures of this invention are useful in the formation of foundrycores. In addition, these procedures can be utilized wherever it isdesirable to bind particles together provided there is sufficientpermeability to allow solvent contact with the resin and subsequentvolatilization.

The specific embodiments in which an exclusive property or privilege isclaimed are defined as follows:

1. The process for the formation of foundry cores or molds, whichcomprises mixing aggregate with a resin, placing the resultant mixturein a mold and compressing said mixture therein, introducing a solventinto said mold and solvating said resin, removing said solvent from saidmold, and removing the resultant molded article from said mold.

2. The process for the formation of foundry cores, which comprisescoating sand with a resin, drying the resultant mixture to afree-flowing state, placing the resultant tree-flowing, resin-coatedsand in a mold and compressing the coated sand therein to form a core,pressurizing said mold with a solvent to solvate said resin, reducingthe pressure on said mold to remove said solvent, and removing theresultant core from said mold.

3. The process for the formation of foundry cores, which comprisesmixing sand with a resin solution, drying the resultant mixture to formfree-flowing, resin-coated sand, placing said coated sand in a mold andcompressing said coated sand therein to form a core, pressurizing saidmold with a solvent to solvate said resin While maintaining said coreunder compression in said mold and subsequently reducing the pressure insaid mold to remove said solvent, and removing the resultant core fromsaid mold.

4. The process according to claim 3, wherein said resin is a syntheticresin which is solvated in said mold to a liquid state with a volatileliquid or gaseous solvent which is thereafter removed under vacuum fromsaid mold.

5. The process according to claim 3, wherein the resincoated sandcomprises 1 to 8 weight percent by weight of the sand of a normallysolid thermosetting resin selected from the group consisting ofhydrocarbon resins, alkyd resins, rosin ester resins, novolak resins,and acrylic 20 7. The process according to claim 5, wherein said resinis a phenol-formaldehyde novolak resin and said solvent is methylenechloride.

8. The process according to claim 5, wherein said resin is aphenol-formaldehyde novolak resin and said solvent is chloroform.

9. The process according to claim 5, wherein said resin is aphenol-formaldehyde novolak resin and said solvent is diethyl ether.

10. The process according to claim 5, wherein said resin is an acrylicresin and said solvent is methylene chloride.

11. The process according to claim 5, wherein said resin is an acrylicresin and said solvent is diethyl ether.

12. The process according to claim 5, wherein said resin is an acrylicresin and said solvent is chloroform.

References Cited by the Examiner UNITED STATES PATENTS 2,154,185 4/1939Robie 264l23 2,517,815 8/1950 Weston 22193 2,583,036 l/ 1952 Wolf 22-194FOREIGN PATENTS 321,967 7/ 1957 Switzerland.

I. SPENCER OVER'HOLSER, Primary Examiner.

MARCUS U. LYONS, Examiner.

3. THE PROCESS FOR THE FORMATION OF FOUNDRY CORES, WHICH COMPRISESMIXING SAND WITH A RESIN SOLUTION, DRYING THE RESULANT MIXTURE TO FORMFREE-FLOW, RESIN-COATED SAND, PLACING SAID COATED SAND IN AMOLD ANDCOMPRESSING SAID COATED SAND THEREIN TO FORM A CORE, PRESSURIZING SAIDMOLD WITH A SOLVENT TO SOLVATE SAID RESIN WHILE MAINTAINING SAID COREUNDER COMPRESSION IN SAID MOLD AND SUBSEQUENTLY REDUCING THE PRESSURE INSAID MOLD REMOVE SAID SOLVENT, AND REMOVING THE RESULTANT CORE FROM SAIDMOLD.